The present invention relates to a method for production of transparent ceramics making use of sintering aids. The invention also relates to the use of specific substances as sintering aids. The ceramics described in this invention are so-called optoceramics.
An optoceramic according to the present invention is understood to be highly transparent material that is essentially single phase, polycrystalline and based on an oxide or a chalcogenide. Optoceramics are thus a special subdivision of ceramics. “Single phase” in this context is understood to mean that at least more than 95% by weight of the material, preferably at least 97% by weight, more preferably 99% by weight and most preferably 99.5 to 99.9% by weight of the material is present in the form of crystals of the desired composition. The individual crystals are densely arranged and, relative to the theoretical density, have densities of at least 99% by weight, preferably at least 99.9% by weight, and more preferably at least 99.99% by weight. Accordingly, the optoceramics are nearly free of pores.
Optoceramics are distinguished from glass ceramics by the fact that glass ceramics comprise high proportions of amorphous glass phase next to crystalline phase. Similarly, the high densities of optoceramics may not be achieved with conventional ceramics. Neither glass ceramics nor conventional ceramics have the advantageous properties of optoceramics like certain refractive indexes, Abbe numbers, values of relative partial dispersion and particularly the advantageous high transparency for light in the visible and/or infrared spectral range.
The optoceramics obtained by carrying out the method of the present invention show improved transmission properties when compared to other transparent ceramics. The transmission properties are characterized in terms of internal transmittance. “Internal transmittance” in the field of optoceramics, unlike the field of glass, describes the percentaged ratio of transmitted light intensity and the maximum light transmission achievable with the respective material. This value is indicative of the quality of the optoceramics in terms of favorably low scattering loss. Therefore, “internal transmittance” in the field of optoceramics describes the loss of light intensity within the ceramic that is due to light scattering at grain boundaries or pores and absorption of impurities. The internal transmittance in accordance with the present invention is determined by measurement of the in-line transmission of the respective ceramic material. Determining in-line transmissions is a standard procedure for the person of ordinary skill in the art.
The present invention especially refers to a method for production of transparent, polycrystalline optoceramics comprising crystal composites with a stoichiometry of A2±xB2±xE7. At least 95% by weight of these crystals have symmetrical, cubic crystal structures. The crystal structures are of the pyrochlore- or fluorite-type. The method of production according to the present invention provides for a cost effective production of optoceramics.
The method for production of optoceramics according to the present invention provides for improved optical properties of the thus obtained material. By making use of certain sintering aids the optoceramics obtained by carrying out the process are essentially free of pores, so that penetrating light is not refracted or scattered at grain boundaries. The optoceramics obtained according to the process of the present invention have similar optical properties as single crystals but are cost-effectively obtainable and moldable.
The prior art describes optoceramics with pyrochlore structure. These are for example disclosed in Ji et al., “Fabrication of transparent La2Hf2O7 ceramics from combustion synthesized powders”, Mat. Res. Bull. 40(3) 553-559 (2005)”. This publication describes use of powders having similar chemical compositions like those of the present invention. The powders in this publication are synthesized by combustion reactions. Making use of this method of production only such ceramics are obtainable, which have transparency values in the region of 70% at a sample thickness of <1 mm. This is not enough for optical applications.